a) Field of the Invention
The present invention relates to a variable displacement pump for use in equipment using pressure fluid, such as a power steering apparatus for alleviating the force for operating the steering wheel of an automobile.
b) Discussion of Related Art
Conventionally, fixed displacement vane pumps which are rotatively driven directly by an automobile engine have generally been used as pumps for power steering apparatuses. With such fixed displacement pumps, however, since the discharge flow rate increases or decreases in correspondence with the revolution rate of the engine, i.e., a driving source, the fixed displacement pumps have characteristics which are mutually contradictory to those of the power steering apparatus that require a large steering-assisting force during a standstill or low-speed running and a small steering-assisting force during high-speed running.
Accordingly, as such a pump, one is used which is capable of securing a discharge flow rate that makes it possible to obtain a required steering-assisting force even during low-speed running when the number of revolutions per unit time is small. At the same time, a flow control valve is required for controlling the discharge flow rate to a fixed level or lower when the number of revolutions has become large. For this reason, with such a pump, the number of component parts used increases, the structure becomes complex, and the structures of passages are also complex, inevitably making the overall apparatus large in size and resulting in higher cost.
In addition, if the flow control valve is used, the discharge flow is recirculated to the tank side, so that there are also problems in that the required driving power becomes large, the energy loss is large, and the oil temperature rises.
To overcome the drawbacks of the fixed displacement pumps, various variable displacement pumps which are capable of reducing the discharge flow rate in a step-wise manner with an increase in the number of revolutions have been proposed, as disclosed in Japanese Patent Application Laid-Open Nos. 53-130505, 56-143383, and 58-93978, and Japanese Utility Model Application Publication No. 63-14078.
Such variable displacement pumps do not require the flow control valve used in the fixed displacement type, prevent a wasteful increase in driving power, and excel in the energy efficiency. Moreover, such variable displacement pumps are capable of reducing the rise in oil temperature since there is no return flow to the tank side, and are capable of preventing problems such as leakage in the pump interior and a decline in the volume efficiency.
The variable displacement pumps disclosed in Japanese Patent Application Laid-Open No. 56-143383 and the like are arranged as follows: A cam ring is arranged movably in a pump casing, a pair of fluid chambers serving as control chambers are formed in a gap formed between the cam ring and the pump casing, the pressure on the upstream and downstream sides of an orifice provided midway in a discharge passage is introduced into the respective chambers, and the differential pressure is made to act directly on the cam ring so as to appropriately move the cam ring against the urging force of a spring so as to change the volume of the pump chamber, thereby effecting appropriate discharge flow-rate control.
With such a conventional pump, however, the cam ring is held in the pump housing in such a manner as to be linearly movable, and is only made to undergo displacement by moving by means of the pressure differential between the upstream and downstream sides of the orifice provided directly or indirectly in the discharge passage. In addition, the numbers of component parts and fluid passages in various sections of the pump have been large, and there have been problems not only in machinability and assembly, but also in reliability operation and durability, so that its feasibility has been poor.
With reference to FIG. 6 and the like, a brief description will be given of an example of the variable-volume type vane pump described above. In the drawing, numeral 1 denotes a pump body; 2 denotes a cam ring which is disposed in an elliptical space 3 formed in the body 1 by means of a pivotally supporting portion 2a, and to which an urging force is imparted in the direction of the shadowed arrow in the drawing; 4 denotes a rotor which is accommodated in the cam ring 2 by being offset toward one side in such a manner as to form a pump chamber 5 on the other side, and which allows vanes 4a to project and to retract, the vanes 4a being held in such a manner as to be capable of radially advancing or retracting as the rotor 4 is rotatively driven by an external driving source.
Incidentally, numeral 4b in the drawing denotes a drive shaft of the rotor 4, and the rotor 4 is rotatively driven in the direction indicated by the arrow in the drawing. Further, numerals 3a and 3b denote passages for introducing control pressure for swinging and displacing the cam ring 2, e.g., fluid pressure or the like on the upstream and downstream sides of a variable orifice provided in a pump discharge-side passage, into chambers formed by being made open to the respective side chambers of the cam ring 2 in the body space 3. The arrangement provided is such that the cam ring 2 is made to undergo displacement by swinging in correspondence with the flow rate on the discharge side of the pump, thereby effecting discharge-side flow-rate control so as to reduce the discharge-side flow rate with an increase in the number of revolutions of the pump.
Numeral 6 denotes a pump suction-side opening which is open in face-to-face relation to a pump suction-side region 5A in the pump chamber 5; numeral 7 denotes a pump discharge-side opening which is open in face-to-face relation to a pump discharge-side region 5B in the pump chamber 5. These openings 6 and 7 are formed by either a pressure plate or a side plate (neither are shown) which are fixed wall portions for holding the pump component elements constituted by the rotor 4 and the cam ring 2 by clamping the same on both sides thereof.
In addition, numerals 8 and 9 denote a pair of fluid chambers which are formed as high-pressure and low-pressure sides, respectively, which are formed on both sides of the outer periphery of the cam ring 2 within the elliptical space 3. The fluid pressure or the like on upstream and downstream sides of the variable orifice in the pump discharge-side passage is introduced into these chambers 8 and 9 through the aforementioned passages 3a and 3b, whereby the cam ring 2 is made to undergo displacement by swinging in a required direction to make the volume of the pump chamber 5 variable, thereby variably controlling the discharge flow rate in correspondence with the flow rate on the discharge side of the pump.
Here, an urging force is imparted to the cam ring 2 from the fluid-pressure chamber 9 side as indicated by F in the drawing, so that the volume of the pump chamber 5 can be maintained at a maximum level at normal times. In addition, numeral 2b in the drawing denotes a seal member provided on the outer periphery of the cam ring 2 so as to define the fluid-pressure chambers 8 and 9 on both sides in association with the pivotally supporting portion 2a provided on the outer periphery of the cam ring 2.
Further, numerals 6a and 7a denote goatee-shaped notches which are formed in such a manner as to continue from terminating portions, in the rotating direction of the pump, of the pump suction-side opening 6 and the discharge-side opening 7. When distal ends of the vanes 4a are brought into sliding contact with the inner periphery of the cam ring 2 as the rotor 4 rotates so as to perform pumping action, these notches 6a and 7a function to allow the fluid pressure to escape gradually from the high-pressure side to the low-pressure side between the space sandwiched by vanes which approach the end portion of each opening 6 and 7 and the space between vanes adjacent thereto. Such notches 6a and 7a are effective in preventing the occurrence of the problem of pulsation in the fluid pressure on the discharge side of the pump due to the occurrence of sharp fluctuations in pressure and surge pressure as the space between the vanes 4a immediately reaches the end of each opening 6 and 7.
With the pump having the above-described construction, the arrangement provided is such that, as the rotor 4 is rotated, a required state of communication of the space defined by the adjacent vanes with each opening 6 and 7 is established after the space is made to communicate through each notch 6a and 7a with each opening 6 and 7, so as to allow the fluid pressure to escape gradually from the high-pressure side to the low-pressure side, and to control sharp fluctuations in pressure in the space between the vanes 4a and reduce the surge pressure, thereby preventing pulsation occurring in the fluid pressure on the discharge side of the pump.
According to the structure of the conventional variable displacement pump described above, there has been a problem in that, at the positions of the pump suction-side opening 6 and the pump discharge-side opening 7 in the pump chamber 5, pulsation on the pump discharge side is large, and the noise level on that side is also large.
This is attributable to the fact that the interior of the pump chamber ceases to be able to undergo precompression when the pump chamber formed between the vanes 4a is made to communicate with the pump discharge-side opening 7, so that this pump chamber is abruptly opened to the high-pressure region, resulting in large pulsation on the discharge side of the pump.
As measures against such a pulsation phenomenon, as shown in FIG. 7, an attempt has been made to offset the discharge-side opening 7 in the pump discharge-side region 5B within the pump chamber 5 by a predetermined angle toward the compression side (the pump suction-side region 5A side) within the pump chamber 5, so as to allow precompression. Also, an attempt has been made to form the goatee-shaped notch 7a such as the one described above in such a manner as to continue from a terminating end, in the rotating direction of the pump, of the pump discharge-side opening 7, i.e., the pump suction-side region 5A side, thereby making it possible to gradually open the pump chamber to the pump discharge-side opening 7.
However, as an issue encountered in devising such a measure, there sometimes arises an unbalanced force in the acting force due to the fluid pressure in the pump discharge-side region 5B since the pump chamber 5 is defined by the cam ring 2 which is swung about the pivotally supporting portion 2a within the pump body 1.
That is to say, as is apparent from FIG. 7, the angular ranges of the pump discharge-side opening 7 corresponding to the left and right fluid-pressure chambers 8 and 9 formed on both sides of the cam ring 2 with respect to the pivotally supporting portion 2a become .alpha. and .alpha.+.beta., so that the pump discharge-side opening 7 which is open in the pump discharge-side region 5B is displaced toward the fluid-pressure chamber 9 which is the low-pressure side. The pump discharge-side pressure corresponding to the angular difference .beta. acts as an unbalanced force which causes the cam ring 2 to undergo swinging displacement rightwards in the drawing.
If the internal pressure at a portion of the cam ring 2 where its outer peripheral side corresponds to the low-pressure side fluid-pressure chamber 9, particularly the pressure within the chamber at a portion corresponding to the angle .beta., rises due to the positional displacement of the pump discharge-side opening 7 which is open in the pump chamber 5, then a force causing the cam ring 2 to swing in the direction indicated by the arrow in the drawing (i.e. a clockwise direction) acts due to the differential pressure in and outside the cam ring 2. Then, if the flow rate of pump discharge decreases due to a reduction in the volume of the pump chamber 5 entailed by such a movement, a problem inevitably arises in that it is difficult to secure the flow rate during loaded operation when the apparatus being used which receives the supply of the pressure fluid from this pump is operated, i.e., when the pump is under a load.
Besides, in Japanese Patent Application No. 4-358801 and the like, pumps having the following structure have been proposed: The cam ring 2 is arranged to be displaceable by moving in correspondence with the change in the number of revolutions of the pump, and in order to obtain a desired pump discharge flow rate by the displacement of the cam ring 2, a changeover valve which is changed over in correspondence with the fluctuations in the flow rate on the discharge side of the pump is provided for the left and right fluid-pressure chambers 8 and 9 defined around the outer periphery of the cam ring 2, so that the fluid pressures controlled to predetermined levels are supplied to the respective chambers by means of the valve.
Fluctuations and the like of the fluid pressure introduced into the left and right fluid-pressure chambers 8 and 9 on the outer periphery of the cam ring 2 in such a variable displacement pump are described below.
Namely, the fluid pressure P.sub.B in the low-pressure side fluid-pressure chamber 9 on the right-hand side in the drawing on the outer periphery of the cam ring 2 is apparent from the diagram shown in FIG. 8, and this fluid pressure P.sub.B is the cam-ring outer-surface pressure corresponding to the cam-ring inner-surface pressure on the right-hand side. Here, such P.sub.B is not completely made to communicate with the pump suction side (drain side) due to the function of the above-described changeover valve even in a flow-rate adjusting region where the number of revolutions of the pump has become large, and a state of a predetermined level of low pressure is maintained.
Meanwhile, the fluid pressure P.sub.A in the high-pressure side fluid-pressure chamber 8 on the left-hand side in the drawing on the outer periphery of the cam ring 2 is apparent from the diagram shown in FIG. 8, and this fluid pressure P.sub.A is the cam-ring outer-surface pressure corresponding to the cam-ring inner-surface pressure on the left-hand side in the drawing. This fluid pressure P.sub.A is slightly larger than the aforementioned P.sub.B in the flow-rate adjusting region. Then, the pressure differential between P.sub.A and P.sub.B at this time corresponds to a spring force F for urging the cam ring 2 leftwards in the drawing, and the fluid pressures P.sub.A and P.sub.B are normally balanced by this spring force F.
In such a relationship of pressure, the inner-surface pressure of the cam ring 2 and the outer-surface pressure of the cam ring in the case where the pump discharge-side opening 7 on the right-hand side of the cam ring 2 is displaced toward the low-pressure side fluid-pressure chamber 9 by an angular difference .beta., as described above, are described below. Here, the pump discharge-side pressure is set as P.
That is, if an unbalanced force acts due to the angular difference .beta. described above, the pressure differential at the low-pressure side fluid-pressure chamber 9 portion is apparent from the diagram in FIG. 8 (pump discharge pressure P-P.sub.B), so that the cam ring 2 is made to undergo displacement by swinging in the direction in which the volume of the pump chamber 5, i.e., the discharge rate, is reduced. In particular, such displacement of the cam ring 2 by swinging in the direction in which the discharge rate is reduced takes place in the flow-rate adjusting region.
In other words, if the cam ring 2 undergoes displacement by swinging and vibration due to the unbalanced force occurring owing to the above-described imbalance of the fluid pressure, large fluctuations in the flow rate occur on the discharge side of the pump. Hence, pulsation becomes large, and presents a problem in the characteristics of the pump, so that it is desirable to overcome such a problem.
In particular, such a problem is noticeable in cases where the fluid pressure rises in the main supply passage due to the operation of the equipment being used to which the fluid pressure from the variable displacement pump is supplied, and large fluctuations occur on the pump discharge-side pressure due to an increase in the pressure differential between the upstream and downstream sides of the metering orifice provided in that passage or midway in the pump discharge-side passage. Thus there has been a need to overcome such a problem.
For instance, in cases where the equipment being used is a power steering apparatus, a large amount of flow or a small amount of flow circulates to the power cylinder side, so that the steering wheel becomes suddenly heavy or light. It is desirable to overcome such instability.
The present invention has been devised in view of such circumstances, and its object is to obtain a variable displacement pump which is capable of eliminating swinging displacement which was liable to occur due to the swinging displacement caused by an unbalanced force occurring in and outside the cam ring, thereby reducing large fluctuations in the flow rate, pulsation and the like on the pump discharge side, and preventing the discharge flow rate from declining.